A high degree of compound separation, selectivity and identification is made possible by combining liquid chromatography techniques with molecular detector methods which provide structural information. This approach has been recognized as extremely valuable for the identification of various components of complex chemical mixtures. Particularly, liquid chromatography (LC) has proven to be excellent means for separating a chemical mixture and for determining the individual constituents, either quantitatively or volumetrically. However, LC devices used by themselves have the disadvantage that they do not satisfactorily identify the separated chemical constituents.
On the other hand, the mass spectrometer (MS) is extremely capable and sensitive in identifying single chemical components, but considerable difficulty is experienced in trying to utilize such equipment in identifying the components of a chemical mixture. Consequently, hybrid techniques, which combine chromatography with molecular methods such as mass spectrometry and Fourier transform infrared spectrometry, have been developed and are used extensively for component analysis of complex chemical mixtures.
The high scan speed and sensitivity of Fourier transform infrared (FTIR) spectroscopy have enabled the recording of infrared spectra of individual components of a mixture which have been separated by chromatographic techniques. Coupling of chromatography with FTIR equipment has been successfully accomplished for gas chromatography (GC). However, many chemical compounds and mixtures are not sufficiently volatile for GC separation. Moreover, the sensitivity of a combination GC/FTIR mechanism is reduced for less volatile compounds, making this combination unacceptable. Particularly, the less volatile and/or more polar compounds in a mixture must usually be separated by LC.
Interfacing of LC mechanisms with FTIR devices has not heretofore been substantially successful due to the infrared absorption of the mobile phase of the LC eluent. Generally, solvents which are good mobile phases for LC applications are also usually strong infrared absorbers. To try to address this problem, two general types of systems have been developed: (1) flow cells which take advantage of some mobile phases which have large infrared (IR) windows; and (2) elimination of the mobile phase prior to deposition of the eluate on an appropriate substrate. Each of these approaches, however, have their own problems in achieving a reliable and universal interface arrangement.
For example, all solvents absorb some infrared radiation, and the degree of such absorption defines the maximum path length which a flow cell can have which will allow identifiable spectra to be obtained. Additionally, mobile phases having large IR windows are generally of low polarity and are used only for normal-phase LC. The shorter path lengths which must be used to minimize interference resulting from mobile phase absorption similarly limit the volume of the flow cell, thereby limiting the concentration of the analyte being measured at any one instant, and thus compromising the accuracy of the process overall. The major challenge of interfacing normal-phase and reverse-phase LC to IR techniques is the incompatibility of typical solvents to identification of unknown constituents by IR technology. Consequently, water and other typical mobile phases used in LC separations are best eliminated prior to measuring the IR spectrum of a component.
A variety of methods and devices have been directed toward eliminating solvents prior to FTIR procedures, including flowing effluent from a capillary LC column into a stainless steel wire net designed to eliminate the solvent as a result of a heated gas flow. In this approach, the sample material is suspended between the metal meshing, and the deposits are then analyzed. Griffiths et al. developed a system wherein the LC effluent is deposited on an IR transparent substrate as warm nitrogen induces solvent evaporation prior to IR analysis. An interface was developed by Gagel and Biemann in which deposition of the sample material was to be continuous and where effluent from a microbore LC was continuously sprayed onto a rotating disk as warm nitrogen was passed across the disk to evaporate the solvent. In that procedure, however, the FTIR spectra were measured off-line by fastening the collection device to a reflectance accessory.
A solvent removal interface developed by Kalasinsky for reverse phase LC contemplated the elimination of water by employing a particular chemical (2,2′-dimethoxypropane) to convert the water to methanol and acetone for deposition on a KCl substrate. Such conversion requires specific matching of chemicals and collection substrates, and does not truly remove the solvent but merely converts it to other substances which can independently add interference to analysis results.
Browner and coworkers developed a monodisperse aerosol generator interface for combining LC and FTIR spectrometry known as the MAGIC interface. With this interface, mobile phase elimination was to be accomplished at room temperature, wherein effluent from an LC enters the interface through a 25 micrometer diameter orifice to form a liquid jet. The jet is dispersed by a Helium (He) stream to create a fine aerosol which is directed from a desolvation chamber into first and second momentum separators. In the first momentum separator, evaporated solvent and Helium are removed by vacuum pumps, and the nonvolatile analyte continues into the second momentum separator where any residual volatile material is to be removed. The nonvolatile analyte is then deposited on a KBr (potassium bromide) window which is removed and placed in a beam condenser for IR analysis. Because the solvent is eliminated prior to deposition on the substrate, the isolated analyte can be deposited on a variety of substrates for various IR detection methods.
In U.S. Pat. Nos. 4,814,612 and 4,883,958, which are incorporated herein by reference, M. L. Vestal et at. described similar apparatuses and methods for coupling LC and solid phase detectors, including the use of thermospray vaporizers which vaporize most of the solvent prior to introduction to a desolvation chamber. The device described in the Vestal '958 patent further contemplates passing the vaporized solvent and added carrier gas through one or more solvent removal chambers, which can remove solvent by condensation or diffusion through a membrane to a counterflowing gas stream. This device may further include a momentum separator to concentrate particles relative to the remaining solvent vapor and carrier gas. This patent also teaches the direction of a particle beam for impact with a cryogenically cooled deposition surface. In the Vestal '612 patent, a moving belt is provided for receiving the particle beam, and a temperature transducer is positioned adjacent the belt to maintain the belt at a temperature where no significant amount of the particle sample will be vaporized, yet warm enough that residual liquid solvent is vaporized efficiently in a stream of counterflowing gas which passes over the belt.
The Vestec Universal Interface incorporates many of the features described in the Vestal patents mentioned above, and was commercially available in the industry from Vestec Corporation of Houston, Tex.
An apparatus combining LC technology with mass spectrometry is described in U.S. Pat. No. 4,980,057, which is incorporated herein by reference, issued to S. B. Dorn, et al. The Dorn '057 device includes a nebulizer which volatilizes the LC eluate to form an aerosol which passes through a desolvation chamber. The nebulizer introduces an inert gas which helps vaporize the solvent and carries the aerosol to a momentum separator which accelerates the particles to sonic velocities. The momentum separator includes three vacuum pumping stages, wherein the first two stages are defined by conical skimmer nozzles, and the third chamber includes a long inlet tube which provides the vacuum pumping restriction. The resulting particle beam is provided to the MS ion source for analysis.
Another approach to this analysis problem is the LC-IR sample delivery system developed by S. Bourne based on ultrasonic nebulization followed by evaporation. Versions of this system have been available from Bourne Scientific, Nicolet and Bio-Rad corporations, as described in U.S. Pat. Nos. 5,045,703; 5,039,614; 5,238,653 and 4,552,723, which are incorporated herein by reference.
LC sample delivery by pneumatic, thermal or ultrasonic nebulization, followed by evaporation and deposition at atmospheric pressure or in a vacuum onto a rotating germanium disk is a technique developed by Lab Connections. The germanium disk is then transferred to, and read by an FTIR, as described in U.S. Pat. Nos. 5,772,964 and 4,823,009, which are incorporated herein by reference.
In U.S. Pat. No. 5,538,643, which is incorporated herein by reference, Kallos describes an LC-FTIR interface sample handling process consisting of nebulizing an LC eluent, removing the solvent by a combination membrane-and-momentum separator, followed by focused deposition onto a cryogenic surface with subsequent thermal manipulation of this surface to remove remaining solvent.
Consequently, while a great number of investigations and techniques have been attempted heretofore, LC/FTIR interfaces have thus far shown only limited success in providing interpretable IR spectra from normal-phase and reverse-phase separations, due to inadequate solvent elimination and/or limited applicability to IR analysis. Although many of these previously developed systems generate aerosols from the liquid stream, and partially desolvate the stream, none of them effectively, reliably and robustly separates the solvent vapor and non-condensable carrier gas from the particulate stream.
These and other deficiencies in or limitations of the prior art are overcome in whole or at least in part by the apparatus and related methods of this invention. As described hereinafter, the present invention successfully and effectively removes interfering materials, thus enabling applications of the particle stream, such as for analysis, which would otherwise be prevented or limited.